Annotation of src/usr.bin/ssh/sshd.8, Revision 1.8
1.1 deraadt 1: .\" -*- nroff -*-
2: .\"
3: .\" sshd.8.in
4: .\"
5: .\" Author: Tatu Ylonen <ylo@cs.hut.fi>
6: .\"
7: .\" Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
8: .\" All rights reserved
9: .\"
10: .\" Created: Sat Apr 22 21:55:14 1995 ylo
11: .\"
1.8 ! markus 12: .\" $Id: sshd.8,v 1.7 1999/10/07 21:45:02 markus Exp $
1.1 deraadt 13: .\"
1.2 deraadt 14: .Dd September 25, 1999
15: .Dt SSHD 8
16: .Os
17: .Sh NAME
18: .Nm sshd
19: .Nd secure shell daemon
20: .Sh SYNOPSIS
21: .Nm sshd
22: .Op Fl diq
23: .Op Fl b Ar bits
24: .Op Fl f Ar config_file
25: .Op Fl g Ar login_grace_time
26: .Op Fl h Ar host_key_file
27: .Op Fl k Ar key_gen_time
28: .Op Fl p Ar port
29: .Sh DESCRIPTION
30: .Nm
1.1 deraadt 31: (Secure Shell Daemon) is the daemon program for
1.2 deraadt 32: .Xr ssh 1 .
1.1 deraadt 33: Together these programs replace rlogin and rsh programs, and
34: provide secure encrypted communications between two untrusted hosts
35: over an insecure network. The programs are intended to be as easy to
36: install and use as possible.
1.2 deraadt 37: .Pp
38: .Nm
1.1 deraadt 39: is the daemon that listens for connections from clients. It is
40: normally started at boot from
1.2 deraadt 41: .Pa /etc/rc .
42: It forks a new
1.1 deraadt 43: daemon for each incoming connection. The forked daemons handle
44: key exchange, encryption, authentication, command execution,
45: and data exchange.
1.2 deraadt 46: .Pp
47: .Nm
48: works as follows. Each host has a host-specific RSA key
1.1 deraadt 49: (normally 1024 bits) used to identify the host. Additionally, when
50: the daemon starts, it generates a server RSA key (normally 768 bits).
51: This key is normally regenerated every hour if it has been used, and
52: is never stored on disk.
1.2 deraadt 53: .Pp
1.1 deraadt 54: Whenever a client connects the daemon, the daemon sends its host
55: and server public keys to the client. The client compares the
56: host key against its own database to verify that it has not changed.
57: The client then generates a 256 bit random number. It encrypts this
58: random number using both the host key and the server key, and sends
59: the encrypted number to the server. Both sides then start to use this
60: random number as a session key which is used to encrypt all further
61: communications in the session. The rest of the session is encrypted
1.5 deraadt 62: using a conventional cipher, currently Blowfish and 3DES, with 3DES
63: being is used by default. The client selects the encryption algorithm
64: to use from those offered by the server.
1.2 deraadt 65: .Pp
1.1 deraadt 66: Next, the server and the client enter an authentication dialog. The
1.2 deraadt 67: client tries to authenticate itself using
68: .Pa .rhosts
69: authentication,
70: .Pa .rhosts
71: authentication combined with RSA host
1.1 deraadt 72: authentication, RSA challenge-response authentication, or password
73: based authentication.
1.2 deraadt 74: .Pp
1.1 deraadt 75: Rhosts authentication is normally disabled
76: because it is fundamentally insecure, but can be enabled in the server
77: configuration file if desired. System security is not improved unless
1.2 deraadt 78: .Xr rshd 8 ,
79: .Xr rlogind 8 ,
80: .Xr rexecd 8 ,
81: and
82: .Xr rexd 8
1.1 deraadt 83: are disabled (thus completely disabling
1.2 deraadt 84: .Xr rlogin 1
1.1 deraadt 85: and
1.2 deraadt 86: .Xr rsh 1
1.1 deraadt 87: into that machine).
1.2 deraadt 88: .Pp
1.1 deraadt 89: If the client successfully authenticates itself, a dialog for
90: preparing the session is entered. At this time the client may request
91: things like allocating a pseudo-tty, forwarding X11 connections,
92: forwarding TCP/IP connections, or forwarding the authentication agent
93: connection over the secure channel.
1.2 deraadt 94: .Pp
1.1 deraadt 95: Finally, the client either requests a shell or execution of a command.
96: The sides then enter session mode. In this mode, either side may send
97: data at any time, and such data is forwarded to/from the shell or
98: command on the server side, and the user terminal in the client side.
1.2 deraadt 99: .Pp
1.1 deraadt 100: When the user program terminates and all forwarded X11 and other
101: connections have been closed, the server sends command exit status to
102: the client, and both sides exit.
1.2 deraadt 103: .Pp
104: .Nm
1.1 deraadt 105: can be configured using command-line options or a configuration
106: file. Command-line options override values specified in the
107: configuration file.
1.2 deraadt 108: .Sh OPTIONS
109: .Bl -tag -width Ds
110: .It Fl b Ar bits
1.1 deraadt 111: Specifies the number of bits in the server key (default 768).
1.2 deraadt 112: .Pp
113: .It Fl d
1.1 deraadt 114: Debug mode. The server sends verbose debug output to the system
115: log, and does not put itself in the background. The server also will
116: not fork and will only process one connection. This option is only
117: intended for debugging for the server.
1.2 deraadt 118: .It Fl f Ar configuration_file
1.1 deraadt 119: Specifies the name of the configuration file. The default is
1.2 deraadt 120: .Pa /etc/sshd_config .
121: .It Fl g Ar login_grace_time
1.1 deraadt 122: Gives the grace time for clients to authenticate themselves (default
123: 300 seconds). If the client fails to authenticate the user within
124: this many seconds, the server disconnects and exits. A value of zero
125: indicates no limit.
1.2 deraadt 126: .It Fl h Ar host_key_file
1.1 deraadt 127: Specifies the file from which the host key is read (default
1.2 deraadt 128: .Pa /etc/ssh_host_key ) .
1.7 markus 129: This option must be given if
130: .Nm
131: is not run as root (as the normal
1.1 deraadt 132: host file is normally not readable by anyone but root).
1.2 deraadt 133: .It Fl i
1.7 markus 134: Specifies that
135: .Nm
136: is being run from inetd.
137: .Nm
138: is normally not run
1.1 deraadt 139: from inetd because it needs to generate the server key before it can
140: respond to the client, and this may take tens of seconds. Clients
141: would have to wait too long if the key was regenerated every time.
1.7 markus 142: However, with small key sizes (e.g. 512) using
143: .Nm
144: from inetd may
1.1 deraadt 145: be feasible.
1.2 deraadt 146: .It Fl k Ar key_gen_time
1.1 deraadt 147: Specifies how often the server key is regenerated (default 3600
148: seconds, or one hour). The motivation for regenerating the key fairly
149: often is that the key is not stored anywhere, and after about an hour,
150: it becomes impossible to recover the key for decrypting intercepted
151: communications even if the machine is cracked into or physically
152: seized. A value of zero indicates that the key will never be regenerated.
1.2 deraadt 153: .It Fl p Ar port
1.1 deraadt 154: Specifies the port on which the server listens for connections
155: (default 22).
1.2 deraadt 156: .It Fl q
1.1 deraadt 157: Quiet mode. Nothing is sent to the system log. Normally the beginning,
158: authentication, and termination of each connection is logged.
1.2 deraadt 159: .El
160: .Sh CONFIGURATION FILE
161: .Nm
1.1 deraadt 162: reads configuration data from
1.2 deraadt 163: .Pa /etc/sshd_config
164: (or the file specified with
165: .Fl f
166: on the command line). The file
167: contains keyword-value pairs, one per line. Lines starting with
168: .Ql #
1.1 deraadt 169: and empty lines are interpreted as comments.
1.2 deraadt 170: .Pp
1.1 deraadt 171: The following keywords are possible.
1.2 deraadt 172: .Bl -tag -width Ds
173: .It Cm AFSTokenPassing
1.3 dugsong 174: Specifies whether an AFS token may be forwarded to the server. Default is
1.2 deraadt 175: .Dq yes .
1.3 dugsong 176: .Pp
1.2 deraadt 177: .It Cm AllowHosts
1.1 deraadt 178: This keyword can be followed by any number of host name patterns,
179: separated by spaces. If specified, login is allowed only from hosts
1.2 deraadt 180: whose name matches one of the patterns.
181: .Ql \&*
182: and
183: .Ql ?
184: can be used as
1.1 deraadt 185: wildcards in the patterns. Normal name servers are used to map the
186: client's host into a canonical host name. If the name cannot be
187: mapped, its IP-address is used as the host name. By default all hosts
188: are allowed to connect.
1.2 deraadt 189: .Pp
1.1 deraadt 190: Note that
1.2 deraadt 191: .Nm
192: can also be configured to use tcp_wrappers using the
1.7 markus 193: .Sy LIBWARP
1.6 aaron 194: compile-time option.
1.8 ! markus 195: .It Cm CheckMail
! 196: Specifies whether
! 197: .Nm
! 198: should check for new mail for interactive logins.
! 199: The default is
! 200: .Dq no .
1.2 deraadt 201: .It Cm DenyHosts
1.1 deraadt 202: This keyword can be followed by any number of host name patterns,
203: separated by spaces. If specified, login is disallowed from the hosts
204: whose name matches any of the patterns.
1.2 deraadt 205: .It Cm FascistLogging
1.1 deraadt 206: Specifies whether to use verbose logging. Verbose logging violates
207: the privacy of users and is not recommended. The argument must be
1.2 deraadt 208: .Dq yes
209: or
210: .Dq no .
211: The default is
212: .Dq no .
213: .It Cm HostKey
1.1 deraadt 214: Specifies the file containing the private host key (default
1.2 deraadt 215: .Pa /etc/ssh_host_key ) .
216: .It Cm IgnoreRhosts
1.1 deraadt 217: Specifies that rhosts and shosts files will not be used in
218: authentication.
1.2 deraadt 219: .Pa /etc/hosts.equiv
1.1 deraadt 220: and
1.2 deraadt 221: .Pa /etc/shosts.equiv
222: are still used. The default is
223: .Dq no .
224: .It Cm KeepAlive
1.1 deraadt 225: Specifies whether the system should send keepalive messages to the
226: other side. If they are sent, death of the connection or crash of one
227: of the machines will be properly noticed. However, this means that
228: connections will die if the route is down temporarily, and some people
229: find it annoying. On the other hand, if keepalives are not send,
1.2 deraadt 230: sessions may hang indefinitely on the server, leaving
231: .Dq ghost
232: users and consuming server resources.
233: .Pp
234: The default is
235: .Dq yes
236: (to send keepalives), and the server will notice
1.1 deraadt 237: if the network goes down or the client host reboots. This avoids
238: infinitely hanging sessions.
1.2 deraadt 239: .Pp
240: To disable keepalives, the value should be set to
241: .Dq no
242: in both the server and the client configuration files.
243: .It Cm KerberosAuthentication
1.1 deraadt 244: Specifies whether Kerberos authentication is allowed. This can
1.7 markus 245: be in the form of a Kerberos ticket, or if
246: .Cm PasswordAuthentication
1.1 deraadt 247: is yes, the password provided by the user will be validated through
1.3 dugsong 248: the Kerberos KDC. Default is
249: .Dq yes .
1.2 deraadt 250: .It Cm KerberosOrLocalPasswd
1.1 deraadt 251: If set then if password authentication through Kerberos fails then
252: the password will be validated via any additional local mechanism
1.2 deraadt 253: such as
254: .Pa /etc/passwd
255: or SecurID. Default is
256: .Dq no .
257: .It Cm KerberosTgtPassing
1.1 deraadt 258: Specifies whether a Kerberos TGT may be forwarded to the server.
1.3 dugsong 259: Default is
260: .Dq no ,
261: as this only works when the Kerberos KDC is actually an AFS kaserver.
1.2 deraadt 262: .It Cm KerberosTicketCleanup
1.7 markus 263: Specifies whether to automatically destroy the user's ticket cache
264: file on logout. Default is
1.3 dugsong 265: .Dq yes .
1.2 deraadt 266: .It Cm KeyRegenerationInterval
1.1 deraadt 267: The server key is automatically regenerated after this many seconds
268: (if it has been used). The purpose of regeneration is to prevent
269: decrypting captured sessions by later breaking into the machine and
270: stealing the keys. The key is never stored anywhere. If the value is
271: 0, the key is never regenerated. The default is 3600
272: (seconds).
1.7 markus 273: .It Cm ListenAddress
274: Specifies what local address
275: .Nm
276: should listen on.
277: The default is to listen to all local addresses.
1.2 deraadt 278: .It Cm LoginGraceTime
1.1 deraadt 279: The server disconnects after this time if the user has not
280: successfully logged in. If the value is 0, there is no time limit.
281: The default is 600 (seconds).
1.2 deraadt 282: .It Cm PasswordAuthentication
1.1 deraadt 283: Specifies whether password authentication is allowed.
1.2 deraadt 284: The default is
285: .Dq yes .
286: .It Cm PermitEmptyPasswords
1.1 deraadt 287: When password authentication is allowed, it specifies whether the
288: server allows login to accounts with empty password strings. The default
1.2 deraadt 289: is
290: .Dq yes .
291: .It Cm PermitRootLogin
1.1 deraadt 292: Specifies whether the root can log in using
1.2 deraadt 293: .Xr ssh 1 .
294: The default is
295: .Dq yes .
296: .Pp
297: Root login with RSA authentication when the
298: .Ar command
299: option has been
1.1 deraadt 300: specified will be allowed regardless of the value of this setting
301: (which may be useful for taking remote backups even if root login is
302: normally not allowed).
1.2 deraadt 303: .It Cm Port
1.1 deraadt 304: Specifies the port number that
1.2 deraadt 305: .Nm
1.1 deraadt 306: listens on. The default is 22.
1.2 deraadt 307: .It Cm PrintMotd
1.1 deraadt 308: Specifies whether
1.2 deraadt 309: .Nm
1.1 deraadt 310: should print
1.2 deraadt 311: .Pa /etc/motd
1.1 deraadt 312: when a user logs in interactively. (On some systems it is also
1.2 deraadt 313: printed by the shell,
314: .Pa /etc/profile ,
315: or equivalent.) The default is
316: .Dq yes .
317: .It Cm QuietMode
1.1 deraadt 318: Specifies whether the system runs in quiet mode. In quiet mode,
319: nothing is logged in the system log, except fatal errors. The default
1.2 deraadt 320: is
321: .Dq no .
322: .It Cm RandomSeed
1.4 deraadt 323: Obsolete. Random number generation uses other techniques.
1.2 deraadt 324: .It Cm RhostsAuthentication
1.1 deraadt 325: Specifies whether authentication using rhosts or /etc/hosts.equiv
326: files is sufficient. Normally, this method should not be permitted
1.7 markus 327: because it is insecure.
328: .Cm RhostsRSAAuthentication
329: should be used
1.1 deraadt 330: instead, because it performs RSA-based host authentication in addition
331: to normal rhosts or /etc/hosts.equiv authentication.
1.2 deraadt 332: The default is
333: .Dq no .
334: .It Cm RhostsRSAAuthentication
1.1 deraadt 335: Specifies whether rhosts or /etc/hosts.equiv authentication together
1.2 deraadt 336: with successful RSA host authentication is allowed. The default is
337: .Dq yes .
338: .It Cm RSAAuthentication
339: Specifies whether pure RSA authentication is allowed. The default is
340: .Dq yes .
341: .It Cm ServerKeyBits
1.1 deraadt 342: Defines the number of bits in the server key. The minimum value is
343: 512, and the default is 768.
1.7 markus 344: .It Cm SkeyAuthentication
345: Specifies whether
346: .Xr skey 1
347: authentication is allowed. The default is
348: .Dq yes .
349: Note that s/key authentication is enabled only if
350: .Cm PasswordAuthentication
351: is allowed, too.
1.2 deraadt 352: .It Cm StrictModes
353: Specifies whether SSH should check file modes and ownership of the
1.1 deraadt 354: user's home directory and rhosts files before accepting login. This
355: is normally desirable because novices sometimes accidentally leave their
1.7 markus 356: directory or files world-writable. The default is
357: .Dq yes .
1.2 deraadt 358: .It Cm SyslogFacility
1.1 deraadt 359: Gives the facility code that is used when logging messages from
1.2 deraadt 360: .Nm sshd .
1.1 deraadt 361: The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2,
362: LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7. The default is DAEMON.
1.2 deraadt 363: .It Cm X11Forwarding
364: Specifies whether X11 forwarding is permitted. The default is
365: .Dq yes .
1.1 deraadt 366: Note that disabling X11 forwarding does not improve security in any
367: way, as users can always install their own forwarders.
1.6 aaron 368: .It Cm X11DisplayOffset
369: Specifies the first display number available for
370: .Nm sshd Ns 's
371: X11 forwarding. This prevents
372: .Nm
373: from interfering with real X11 servers.
1.2 deraadt 374: .El
375: .Sh LOGIN PROCESS
1.1 deraadt 376: When a user successfully logs in,
1.2 deraadt 377: .Nm
1.1 deraadt 378: does the following:
1.2 deraadt 379: .Bl -enum -offset indent
380: .It
1.1 deraadt 381: If the login is on a tty, and no command has been specified,
382: prints last login time and
1.2 deraadt 383: .Pa /etc/motd
1.1 deraadt 384: (unless prevented in the configuration file or by
1.2 deraadt 385: .Pa $HOME/.hushlogin ;
386: see the
387: .Sx FILES
388: section).
389: .It
1.1 deraadt 390: If the login is on a tty, records login time.
1.2 deraadt 391: .It
392: Checks
393: .Pa /etc/nologin ;
394: if it exists, prints contents and quits
1.1 deraadt 395: (unless root).
1.2 deraadt 396: .It
1.1 deraadt 397: Changes to run with normal user privileges.
1.2 deraadt 398: .It
1.1 deraadt 399: Sets up basic environment.
1.2 deraadt 400: .It
401: Reads
402: .Pa $HOME/.ssh/environment
403: if it exists.
404: .It
1.1 deraadt 405: Changes to user's home directory.
1.2 deraadt 406: .It
407: If
408: .Pa $HOME/.ssh/rc
409: exists, runs it; else if
410: .Pa /etc/sshrc
411: exists, runs
412: it; otherwise runs xauth. The
413: .Dq rc
414: files are given the X11
1.1 deraadt 415: authentication protocol and cookie in standard input.
1.2 deraadt 416: .It
1.1 deraadt 417: Runs user's shell or command.
1.2 deraadt 418: .El
419: .Sh AUTHORIZED_KEYS FILE FORMAT
1.1 deraadt 420: The
1.2 deraadt 421: .Pa $HOME/.ssh/authorized_keys
1.1 deraadt 422: file lists the RSA keys that are
423: permitted for RSA authentication. Each line of the file contains one
1.2 deraadt 424: key (empty lines and lines starting with a
425: .Ql #
426: are ignored as
1.1 deraadt 427: comments). Each line consists of the following fields, separated by
428: spaces: options, bits, exponent, modulus, comment. The options field
429: is optional; its presence is determined by whether the line starts
430: with a number or not (the option field never starts with a number).
431: The bits, exponent, modulus and comment fields give the RSA key; the
432: comment field is not used for anything (but may be convenient for the
433: user to identify the key).
1.2 deraadt 434: .Pp
1.1 deraadt 435: Note that lines in this file are usually several hundred bytes long
436: (because of the size of the RSA key modulus). You don't want to type
437: them in; instead, copy the
1.2 deraadt 438: .Pa identity.pub
1.1 deraadt 439: file and edit it.
1.2 deraadt 440: .Pp
1.1 deraadt 441: The options (if present) consists of comma-separated option
442: specifications. No spaces are permitted, except within double quotes.
443: The following option specifications are supported:
1.2 deraadt 444: .Bl -tag -width Ds
445: .It Cm from="pattern-list"
1.1 deraadt 446: Specifies that in addition to RSA authentication, the canonical name
447: of the remote host must be present in the comma-separated list of
448: patterns ('*' and '?' serve as wildcards). The list may also contain
449: patterns negated by prefixing them with '!'; if the canonical host
450: name matches a negated pattern, the key is not accepted. The purpose
451: of this option is to optionally increase security: RSA authentication
452: by itself does not trust the network or name servers or anything (but
453: the key); however, if somebody somehow steals the key, the key
454: permits an intruder to log in from anywhere in the world. This
455: additional option makes using a stolen key more difficult (name
456: servers and/or routers would have to be compromised in addition to
457: just the key).
1.2 deraadt 458: .It Cm command="command"
1.1 deraadt 459: Specifies that the command is executed whenever this key is used for
460: authentication. The command supplied by the user (if any) is ignored.
461: The command is run on a pty if the connection requests a pty;
462: otherwise it is run without a tty. A quote may be included in the
463: command by quoting it with a backslash. This option might be useful
464: to restrict certain RSA keys to perform just a specific operation. An
465: example might be a key that permits remote backups but nothing
466: else. Notice that the client may specify TCP/IP and/or X11
467: forwardings unless they are explicitly prohibited.
1.2 deraadt 468: .It Cm environment="NAME=value"
1.1 deraadt 469: Specifies that the string is to be added to the environment when
470: logging in using this key. Environment variables set this way
471: override other default environment values. Multiple options of this
472: type are permitted.
1.2 deraadt 473: .It Cm no-port-forwarding
1.1 deraadt 474: Forbids TCP/IP forwarding when this key is used for authentication.
475: Any port forward requests by the client will return an error. This
1.2 deraadt 476: might be used, e.g., in connection with the
477: .Cm command
1.1 deraadt 478: option.
1.2 deraadt 479: .It Cm no-X11-forwarding
1.1 deraadt 480: Forbids X11 forwarding when this key is used for authentication.
481: Any X11 forward requests by the client will return an error.
1.2 deraadt 482: .It Cm no-agent-forwarding
1.1 deraadt 483: Forbids authentication agent forwarding when this key is used for
484: authentication.
1.2 deraadt 485: .It Cm no-pty
1.1 deraadt 486: Prevents tty allocation (a request to allocate a pty will fail).
1.2 deraadt 487: .El
488: .Ss Examples
1.1 deraadt 489: 1024 33 12121.\|.\|.\|312314325 ylo@foo.bar
1.2 deraadt 490: .Pp
1.1 deraadt 491: from="*.niksula.hut.fi,!pc.niksula.hut.fi" 1024 35 23.\|.\|.\|2334 ylo@niksula
1.2 deraadt 492: .Pp
1.1 deraadt 493: command="dump /home",no-pty,no-port-forwarding 1024 33 23.\|.\|.\|2323 backup.hut.fi
1.2 deraadt 494: .Sh SSH_KNOWN_HOSTS FILE FORMAT
1.1 deraadt 495: The
1.2 deraadt 496: .Pa /etc/ssh_known_hosts
1.1 deraadt 497: and
1.2 deraadt 498: .Pa $HOME/.ssh/known_hosts
1.1 deraadt 499: files contain host public keys for all known hosts. The global file should
500: be prepared by the admistrator (optional), and the per-user file is
501: maintained automatically: whenever the user connects an unknown host
1.6 aaron 502: its key is added to the per-user file.
1.2 deraadt 503: .Pp
1.1 deraadt 504: Each line in these files contains the following fields: hostnames,
505: bits, exponent, modulus, comment. The fields are separated by spaces.
1.2 deraadt 506: .Pp
1.1 deraadt 507: Hostnames is a comma-separated list of patterns ('*' and '?' act as
508: wildcards); each pattern in turn is matched against the canonical host
509: name (when authenticating a client) or against the user-supplied
510: name (when authenticating a server). A pattern may also be preceded
1.2 deraadt 511: by
512: .Ql !
513: to indicate negation: if the host name matches a negated
1.1 deraadt 514: pattern, it is not accepted (by that line) even if it matched another
515: pattern on the line.
1.2 deraadt 516: .Pp
1.1 deraadt 517: Bits, exponent, and modulus are taken directly from the host key; they
1.2 deraadt 518: can be obtained, e.g., from
519: .Pa /etc/ssh_host_key.pub .
1.1 deraadt 520: The optional comment field continues to the end of the line, and is not used.
1.2 deraadt 521: .Pp
522: Lines starting with
523: .Ql #
524: and empty lines are ignored as comments.
525: .Pp
1.1 deraadt 526: When performing host authentication, authentication is accepted if any
527: matching line has the proper key. It is thus permissible (but not
528: recommended) to have several lines or different host keys for the same
529: names. This will inevitably happen when short forms of host names
530: from different domains are put in the file. It is possible
531: that the files contain conflicting information; authentication is
532: accepted if valid information can be found from either file.
1.2 deraadt 533: .Pp
1.1 deraadt 534: Note that the lines in these files are typically hundreds of characters
535: long, and you definitely don't want to type in the host keys by hand.
1.6 aaron 536: Rather, generate them by a script
1.1 deraadt 537: or by taking
1.2 deraadt 538: .Pa /etc/ssh_host_key.pub
1.1 deraadt 539: and adding the host names at the front.
1.2 deraadt 540: .Ss Examples
1.1 deraadt 541: closenet,closenet.hut.fi,.\|.\|.\|,130.233.208.41 1024 37 159.\|.\|.93 closenet.hut.fi
1.2 deraadt 542: .Sh FILES
543: .Bl -tag -width Ds
544: .It Pa /etc/sshd_config
1.1 deraadt 545: Contains configuration data for
1.2 deraadt 546: .Nm sshd .
1.1 deraadt 547: This file should be writable by root only, but it is recommended
548: (though not necessary) that it be world-readable.
1.2 deraadt 549: .It Pa /etc/ssh_host_key
1.7 markus 550: Contains the private part of the host key.
1.1 deraadt 551: This file should only be owned by root, readable only by root, and not
552: accessible to others.
1.2 deraadt 553: .It Pa /etc/ssh_host_key.pub
1.7 markus 554: Contains the public part of the host key.
555: This file should be world-readable but writable only by
1.1 deraadt 556: root. Its contents should match the private part. This file is not
557: really used for anything; it is only provided for the convenience of
558: the user so its contents can be copied to known hosts files.
1.7 markus 559: These two files are created using
560: .Xr ssh-keygen 1 .
1.2 deraadt 561: .It Pa /var/run/sshd.pid
562: Contains the process ID of the
563: .Nm
1.1 deraadt 564: listening for connections (if there are several daemons running
565: concurrently for different ports, this contains the pid of the one
566: started last). The contents of this file are not sensitive; it can be
567: world-readable.
1.2 deraadt 568: .It Pa $HOME/.ssh/authorized_keys
1.1 deraadt 569: Lists the RSA keys that can be used to log into the user's account.
570: This file must be readable by root (which may on some machines imply
571: it being world-readable if the user's home directory resides on an NFS
572: volume). It is recommended that it not be accessible by others. The
573: format of this file is described above.
1.6 aaron 574: .It Pa /etc/ssh_known_hosts
575: This file is consulted when using rhosts with RSA host
1.1 deraadt 576: authentication to check the public key of the host. The key must be
1.6 aaron 577: listed in this file to be accepted.
578: .It Pa $HOME/.ssh/known_hosts
579: The client uses this file
580: and
581: .Pa /etc/ssh_known_hosts
582: to verify that the remote host is the one we intended to
583: connect. These files should be writable only by root/the owner.
1.2 deraadt 584: .Pa /etc/ssh_known_hosts
585: should be world-readable, and
586: .Pa $HOME/.ssh/known_hosts
587: can but need not be world-readable.
1.6 aaron 588: .It Pa /etc/nologin
1.1 deraadt 589: If this file exists,
1.2 deraadt 590: .Nm
1.1 deraadt 591: refuses to let anyone except root log in. The contents of the file
592: are displayed to anyone trying to log in, and non-root connections are
593: refused. The file should be world-readable.
1.6 aaron 594: .It Pa $HOME/.rhosts
1.1 deraadt 595: This file contains host-username pairs, separated by a space, one per
596: line. The given user on the corresponding host is permitted to log in
597: without password. The same file is used by rlogind and rshd.
1.6 aaron 598: The file must
1.1 deraadt 599: be writable only by the user; it is recommended that it not be
600: accessible by others.
1.2 deraadt 601: .Pp
1.1 deraadt 602: If is also possible to use netgroups in the file. Either host or user
603: name may be of the form +@groupname to specify all hosts or all users
604: in the group.
1.2 deraadt 605: .It Pa $HOME/.shosts
606: For ssh,
607: this file is exactly the same as for
608: .Pa .rhosts .
609: However, this file is
610: not used by rlogin and rshd, so using this permits access using SSH only.
611: .Pa /etc/hosts.equiv
612: This file is used during
613: .Pa .rhosts
614: authentication. In the
1.1 deraadt 615: simplest form, this file contains host names, one per line. Users on
616: those hosts are permitted to log in without a password, provided they
617: have the same user name on both machines. The host name may also be
618: followed by a user name; such users are permitted to log in as
1.2 deraadt 619: .Em any
620: user on this machine (except root). Additionally, the syntax
621: .Dq +@group
622: can be used to specify netgroups. Negated entries start with
623: .Ql \&- .
624: .Pp
1.1 deraadt 625: If the client host/user is successfully matched in this file, login is
626: automatically permitted provided the client and server user names are the
627: same. Additionally, successful RSA host authentication is normally
628: required. This file must be writable only by root; it is recommended
629: that it be world-readable.
1.2 deraadt 630: .Pp
1.6 aaron 631: .Sy "Warning: It is almost never a good idea to use user names in"
1.2 deraadt 632: .Pa hosts.equiv .
1.1 deraadt 633: Beware that it really means that the named user(s) can log in as
1.2 deraadt 634: .Em anybody ,
1.1 deraadt 635: which includes bin, daemon, adm, and other accounts that own critical
636: binaries and directories. Using a user name practically grants the
637: user root access. The only valid use for user names that I can think
638: of is in negative entries.
1.2 deraadt 639: .Pp
640: Note that this warning also applies to rsh/rlogin.
641: .It Pa /etc/shosts.equiv
1.1 deraadt 642: This is processed exactly as
1.2 deraadt 643: .Pa /etc/hosts.equiv .
1.1 deraadt 644: However, this file may be useful in environments that want to run both
1.2 deraadt 645: rsh/rlogin and ssh.
1.6 aaron 646: .It Pa $HOME/.ssh/environment
1.1 deraadt 647: This file is read into the environment at login (if it exists). It
1.2 deraadt 648: can only contain empty lines, comment lines (that start with
649: .Ql # ) ,
1.6 aaron 650: and assignment lines of the form name=value. The file should be writable
651: only by the user; it need not be readable by anyone else.
1.2 deraadt 652: .It Pa $HOME/.ssh/rc
1.1 deraadt 653: If this file exists, it is run with /bin/sh after reading the
654: environment files but before starting the user's shell or command. If
655: X11 spoofing is in use, this will receive the "proto cookie" pair in
1.2 deraadt 656: standard input (and
657: .Ev DISPLAY
658: in environment). This must call
659: .Xr xauth 1
660: in that case.
661: .Pp
1.1 deraadt 662: The primary purpose of this file is to run any initialization routines
663: which may be needed before the user's home directory becomes
664: accessible; AFS is a particular example of such an environment.
1.2 deraadt 665: .Pp
1.1 deraadt 666: This file will probably contain some initialization code followed by
667: something similar to: "if read proto cookie; then echo add $DISPLAY
668: $proto $cookie | xauth -q -; fi".
1.2 deraadt 669: .Pp
670: If this file does not exist,
671: .Pa /etc/sshrc
672: is run, and if that
1.1 deraadt 673: does not exist either, xauth is used to store the cookie.
1.2 deraadt 674: .Pp
1.1 deraadt 675: This file should be writable only by the user, and need not be
676: readable by anyone else.
1.2 deraadt 677: .It Pa /etc/sshrc
678: Like
679: .Pa $HOME/.ssh/rc .
680: This can be used to specify
1.1 deraadt 681: machine-specific login-time initializations globally. This file
682: should be writable only by root, and should be world-readable.
1.2 deraadt 683: .Sh AUTHOR
1.1 deraadt 684: Tatu Ylonen <ylo@cs.hut.fi>
1.2 deraadt 685: .Pp
1.1 deraadt 686: Information about new releases, mailing lists, and other related
1.2 deraadt 687: issues can be found from the SSH WWW home page:
688: .Pp
689: .Dl http://www.cs.hut.fi/ssh.
1.5 deraadt 690: .Pp
691: This version of
692: .Nm
693: is a derivative of the original 1.2.12 release, but with bugs removed and
694: newer features re-added. Rapidly after the 1.2.12 release, newer versions
695: bore successively more restrictive licenses. In this version, all components
696: of a restrictive nature (ie. patents) have been directly removed from the
697: source code; any licensed or patented components are chosen from external
698: libraries. The libraries described in
699: .Xr ssl 8
700: are required for proper operation.
1.2 deraadt 701: .Sh SEE ALSO
702: .Xr rlogin 1 ,
703: .Xr rsh 1 ,
704: .Xr scp 1 ,
705: .Xr ssh 1 ,
1.5 deraadt 706: .Xr ssh-add 1 ,
1.2 deraadt 707: .Xr ssh-agent 1 ,
1.5 deraadt 708: .Xr ssh-keygen 1 ,
709: .Xr ssl 8
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